A fuel assembly commonly used in a pressurized water reactor comprises, in general, an upper and lower nozzle facing each other with a space therebetween, a plurality of hollow guide tubes for control rods extending parallel to and spaced apart from one another between these nozzles, both ends of the guide tubes being secured to the nozzles, a plurality of fuel rod support grids mounted on the guide tubes and disposed so as to be spaced apart from one another along the length of the tubes, and a plurality of fuel rods extending through and supported by these fuel rod support grids, the fuel rods extending in parallel to and spaced apart from one another. The fuel rods are arrayed with spaces between one another in orthogonal directions, that is, in the row and line direction of the grid. Thus, the fuel assembly is called a 17.times.17, 15.times.15, etc. type according to the number of the lines and rows.
Further, such fuel assemblies are positioned and loaded on a lower core plate in a nuclear reactor vessel while the upper portion thereof is held by an upper core plate. Thus, during operation of the nuclear reactor, since the reactor coolant after flowing in through many flow holes in the lower core plate, passes through a lower nozzle, flows upwards along the fuel rods, and further flows upwards through the upper nozzle, the fuel assembly experiences an upward coolant flow force. On the other hand, a thermal expansion difference is generated between an in-core structure including the upper and lower core plate and the fuel assembly; and further, the fuel assembly grows or its length increases from the exposure to neutrons. Accordingly, a hold-down spring is fitted onto the upper nozzle and used to accommodate changes in length such as thermal expansion differential, etc. and for holding the fuel assembly at its designed position against the coolant flow force.
A conventional structure of a hold-down spring used in a so-called 17.times.17 type fuel assembly is shown in FIG. 4. In the drawing, a hold-down spring 1 is a composite spring comprising two lower springs 3 and one upper spring 5 and the base ends thereof are fastened with a fitting bolt 7 onto an upper surface of the upper nozzle 9 of the before mentioned fuel assembly. As seen in the detailed drawing of FIG. 5, at a distal end portion of the lower springs 3 a slot 3a extending in a lateral direction is cut out, through which a vertically oriented portion 5a of the upper spring 5 extends. The vertically oriented portion 5a of the upper spring 5 is connected through a bent portion 5b to a main body thereof and an abutting ledge 5c is shaped at the lower end of the bent portion 5b. Thus, the upper spring 5 which can bend in a vertical direction comes into contact with an upper surface of a lower-spring 3 at the abutting ledge 5c after an initial deformation, and thereafter the upper spring 5 and the lower springs 3 make their deformation as one structure. In other words, the hold-down spring 1 has non-linear characteristics while the upper spring 5 has plastic spring characteristics as illustrated in FIG. 6. Such spring characteristics are employed after allowing for increases in the overall length of the fuel assembly accompanying an increased burn-up of the fuel assembly or accumulated operating hours, and thermal expansion differential between an in-core structure and the fuel assembly during operation (hot state). In-addition, considering the operating environment and the stress resistance needed, a precipitation hardened nickel base alloy is used as the material for the hold-down spring 1.